In solar energy collecting devices, planar or curved specular surfaces are commonly arranged to direct the sun's rays to a desired location, e.g., toward a tube containing water. The effectiveness of this type of device depends heavily on the efficiency of reflection, and it is essential that the reflecting surfaces maintain as high a degree of specularity as possible. One way of attaining a high degree of specularity is through vapor-deposition of suitable metals--e.g., gold, silver, copper, or especially, because of its low cost and ease of application, aluminum--on the surface of a smooth polymeric foil. The metalized foil is then either adhered to a planar or suitably contoured surface or, alternatively, mounted or gripped at the edges and a vacuum applied to the center to generate a parabolic surface.
When metalized foils of the type just described are exposed to normal atmospheric conditions, the dew, rain, heat, and abrasion deluster, corrode, oxidize and transparentize certain portions of the originally specular surface. While it might seem reasonable to apply a thin layer of a suitable transparent protective material over the surface of the metal, it is believed that, prior to the present invention, this has never been accomplished successfully. Some polymeric coatings applied to the metal surface aggravate the corrosion process, either because the pH of the coating is initially too far removed from neutral or because corrosive by-products are eliminated from the coating as it ages. Other coatings, which are capable of maintaining satisfactory neutrality over long periods of time, either adhere poorly to the surface of the metal or, for reasons that are not completely clear, cause the metal to separate from the polymeric foil backing.